The invention lies in the fields of thermodynamics and power generation. The invention relates, more specifically, to a continuous-flow steam generator having a combustion chamber for fossil fuel, which is followed on the fuel-gas side, via a horizontal flue, by a vertical gas flue, the containment walls of the combustion chamber being formed from vertically arranged evaporator tubes welded to one another in a gastight manner.
In a power plant with a steam generator, the energy content of a fuel is utilized for the evaporation of a flow medium in the steam generator. The flow medium is thereby conventionally carried in an evaporator circuit. The steam supplied by the steam generator may, in turn, be provided, for example, for driving a steam turbine and/or for a connected external process. When the steam drives a steam turbine, a generator or a working machine is normally operated via the turbine shaft of the steam turbine. Where a generator is concerned, the current generated by the generator may be provided for feeding into an interconnected network and/or an isolated network.
The steam generator may thereby be designed as a continuous-flow steam generator, also referred to as a once-through generator. A continuous-flow steam generator is known from the paper xe2x80x9cVerdampferkonzepte fur Benson-Dampferzeugerxe2x80x9d [xe2x80x9cEvaporator concepts for Benson steam generatorsxe2x80x9d] by Franke, Kxc3x6hler, and Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, pages 352-60. In a continuous-flow steam generator, the heating of steam generator tubes provided as evaporator tubes leads to an evaporation of the flow medium in the steam generator tubes in a single pass.
Continuous-flow steam generators are conventionally designed with a combustion chamber in a vertical form of construction. This means that the combustion chamber is designed for the heating medium or fuel gas to flow through in an approximately vertical direction. The combustion chamber may thereby be followed on the fuel-gas side by a horizontal gas flue, a deflection of the fuel-gas stream into an approximately horizontal flow direction taking place at the transition from the combustion chamber into the horizontal gas flue. However, in general, on account of the thermally induced changes in length of the combustion chamber, combustion chambers of this type require a scaffold on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the manufacture and assembly of the continuous-flow steam generator. The outlay is all the greater, the greater the overall height of the continuous-flow steam generator. This occurs particularly in the case of continuous-flow steam generators which are configured for a steam power output of more than 80 kg/s under full load.
A continuous-flow steam generator is not subject to any pressure limitation, so that it is possible to have live-steam pressures well above the critical pressure of water (pcri=221 bar) where there is still only a slight density difference between liquid-phase and steam-phase medium. A high live-steam pressure is conducive to high thermal efficiency and therefore to low CO2 emissions of a fossil-fired power station which may be fired, for example, with (hard) coal or else with lignite (brown coal) as fuel.
A particular problem is presented by the design of the containment wall of the gas flue or combustion chamber of the continuous-flow steam generator with regard to the tube-wall or material temperatures which occur there. In the subcritical pressure range up to about 200 bar, the temperature of the containment wall of the combustion chamber is determined essentially by the height of the saturation temperature of water when wetting of the inner surface of the evaporator tubes can be ensured. This is achieved, for example, by the use of evaporator tubes which have a surface structure on their inside. For this purpose, in particular, internally ribbed evaporator tubes may be considered, for which the use in a continuous-flow steam generator is known, for example, from the abovementioned paper. These so-called ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transfer from the tube inner wall to the flow medium.
Experience has shown that it is not possible to avoid the situation where the containment wall of the combustion chamber is heated to a differing extent. Due to the different heating of the evaporator tubes, the outlet temperatures of the flow medium from evaporator tubes heated to a greater extent may therefore, in the case of continuous-flow steam generators, be generally higher than where evaporator tubes heated to a normal or lesser extent are concerned. Temperature differences between adjacent evaporator tubes may thereby arise, leading to thermal stresses which may reduce the useful life of the continuos-flow steam generator or may even cause tube cracks.
The object of the present invention is to provide a fossil-fired continuous-flow steam generator of the abovementioned type which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which requires a particularly low outlay in terms of manufacture and assembly and, moreover, during the operation of which temperature differences between adjacent evaporator tubes of the combustion chamber are kept particularly low. It is a further object to provide a continuous-flow steam generator which is especially easy to produce and assemble.
With the above and other objects in view there is provided, in accordance with the invention, a continuous-flow steam generator, comprising:
a combustion chamber having a plurality of burners for fossil fuel and having a fuel-gas side;
a horizontal gas flue substantially at a level with said burners and a vertical gas flue following said combustion chamber on said fuel-gas side;
said combustion chamber having containment walls formed from substantially vertically arranged evaporator tubes welded to one another in a gastight manner, and including a plurality of evaporator tubes each formed with inner ribs defining a multiple thread;
a common inlet header system for a flow medium connected in common to a number of said evaporator tubes of said combustion chamber and a common outlet header system connected in common to said evaporator tubes, such that the number of said evaporator tubes can be acted upon in parallel by the flow medium;
wherein a quotient formed from a steam power output (given in kg/s) under full load of the continuous-flow steam generator and a sum (given in m2) of an inner cross-sectional area of said number of said evaporator tubes capable of being acted upon in parallel by the flow medium is smaller than 1350 (given in kg/sm2).
In other words, the object of the invention are achieved with the continuous-flow steam generator that has a combustion chamber with a number of burners arranged level with the horizontal gas flue and designed in such a way that, in each case for a number of evaporator tubes capable of being acted upon in parallel by flow medium, the quotient formed from the steam power output M (given in kg/s) under full load and the sum of the inner cross-sectional areas A (given in m2) of these evaporator tubes capable of being acted upon in parallel by flow medium is smaller than 1350 (given in kg/sm2.)
The invention proceeds from the notion that a continuous-flow steam generator capable of being produced at a particularly low outlay in terms of manufacture and assembly should have a suspension structure capable of being executed by simple means. A scaffold to be produced at a comparatively low technically outlay for the suspension of the combustion chamber may at the same time be accompanied by a particularly low overall height of the continuous-flow steam generator. A particularly low overall height of the continuous-flow steam generator can be achieved by designing the combustion chamber in a horizontal form of construction. For this purpose, the burners are arranged level with the horizontal gas flue in the combustion chamber wall. Thus, when the continuous-flow steam generator is in operation, the fuel gas flows through the combustion chamber in approximately horizontal main flow direction.
In the case of a horizontal combustion chamber, however, when the continuous-flow steam generator is in operation the rear region of the combustion chamber, as seen on the fuel-gas side, is heated to a comparatively lesser extent than the front region of the combustion chamber, as seen on the fuel-gas side. Moreover, for example, an evaporator tube in proximity to a burner is heated to a greater extent than an evaporator tube arranged in a corner of the combustion chamber. Under these circumstances, in an extreme case, the heating may be about three times greater in the front region of the combustion chamber than in the rear region. In the context of the hitherto customary mass flow densities in the evaporator tubes, given in kg/m2s and in respect of 100% steam power output (full load), of 2000 kg/m2s, the mass throughput decreases in a tube heated to a greater extent and increases in a tube heated to a lesser extent, in each case with respect to the average value of the mass throughput of all the tubes. This behavior is caused by the relatively high proportion of the frictional pressure loss in the total pressure drop of the evaporator tubes. Furthermore, because of the particularly low height of the combustion chamber, the relative differences in length of the evaporator tubes are appreciably greater than where a vertical combustion chamber is concerned. This additionally increases the differences in the heating and in the frictional pressure loss of the individual evaporator tubes. In order nevertheless to ensure approximately identical temperatures between adjacent evaporator tubes, the continuous-flow steam generator should be designed in such a way that a higher throughput of the flow medium is established automatically in an evaporator tube heated to a comparatively greater extent than in an evaporator tube heated to a comparatively lesser extent. This is generally the case when the geodetic pressure drop xcex94pG (given in bar) of an evaporator tube with average heating amounts to a multiple of its frictional pressure loss xcex94pR (given in bar) . The condition for an increase in throughput in an evaporator tube heated to a comparatively greater extent in the case of a constant mass flow is:             (                        Δ          ⁢                      (                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  p                  G                                            +                              Δ                ⁢                                  xe2x80x83                                ⁢                                  p                  R                                            +                              Δ                ⁢                                  xe2x80x83                                ⁢                                  p                  B                                                      )                                    Δ          ⁢                      xe2x80x83                    ⁢          Q                    )              M      =      constant        =      K     less than     0  
where xcex94pB (given in bar) is a change in the acceleration pressure drop, xcex94Q (given in kJ/s) is a change in the heating, M (given in kg/s) is the mass flow and K (given in (bar s)/kJ) is a constant. The condition formulated in this inequality states that, in the case of a constant mass flow, the total pressure loss xcex94(xcex94pG+xcex94xcfx81R=xcex94pB) (given in bar) must decrease under greater heating xcex94Q, that is to say must be mathematically negative. Hence, when the same total pressure loss prevails in a number of evaporator tubes, then the throughput of the flow medium must increase, according to the above-mentioned inequality, in an evaporator tube heated to a greater extent, as compared with an evaporator tube heated to a lesser extent.
Comprehensive calculations, then, have surprisingly yielded the fact that the condition formulated in the inequation is satisfied for continuous-flow steam generators with a horizontal combustion chamber when, for a number of evaporator tubes connected in parallel, the quotient of the steam power output M (given in kg/s) of the continuous-flow steam generator under full load and of the sum of the inner cross-sectional areas A (given in m2) of these evaporator tubes connected in parallel is no higher than 1350 (given in kg/sm2). Hence, formulated mathematically:
M/A less than 1350.
In this case, the steam power output M under the full load of the continuous-flow steam generator is also designated as the permissible steam generation or as the boiler maximum continuous rating (BMCR), and the respective inner cross-sectional area of an evaporator tube is with respect to a horizontal section.
Advantageously, in each case a number of evaporator tubes of the combustion chamber which are connected in parallel are preceded by a common inlet header system and followed by a common outlet header system for flow medium. To be precise, a continuous-flow steam generator produced in this design allows reliable pressure compensation between a number of evaporator tubes connected in parallel, so that in each case all the evaporator tubes connected in parallel have the same total pressure loss. This means that the throughput must increase according to the abovementioned inequation in the case of an evaporator tube heated to a greater extent, as compared with an evaporator tube heated to a lesser extent.
The evaporator tubes of the end wall of the combustion chamber advantageously precede on the flow-medium side the evaporator tubes of the containment walls which form the side walls of the combustion chamber. Particularly beneficial cooling of the highly heated end wall of the combustion chamber is thereby ensured.
In a further advantageous refinement of the invention, the tube inside diameter of a number of the evaporator tubes of the combustion chamber is selected as a function of the respective position of the evaporator tubes in the combustion chamber. The evaporator tubes in the combustion chamber can thereby be adapted to a heating profile predeterminable on the fuel-gas side. The influence thus exerted on the flow through the evaporator tubes keeps temperature differences at the outlet of the evaporator tubes of the combustion chamber low in a particularly reliable way.
For particularly good transmission of the heat of the combustion chamber to the flow medium carried in the evaporator tubes, a number of evaporator tubes advantageously have on their inside in each case ribs forming a multiple thread. In this case, advantageously, a pitch angle xcex1 between a plane perpendicular to the tube axis and the flanks of the ribs arranged on the tube inside is smaller than 60xc2x0, preferably smaller than 55xc2x0.
To be precise, in a heated evaporator tube designed as an evaporator tube without internal ribbing, a so-called smooth tube, it is no longer possible, from a specific steam content onward, to maintain the wetting of the tube wall necessary for particularly good heat transmission. When wetting is absent, there may be a tube wall which is dry in places. The transition to a dry tube wall of this type leads to a so-called heat transmission crisis with an impaired heat transmission behavior, so that, in general, the tube-wall temperatures rise particularly sharply at this point. In an internally ribbed evaporator tube, however, as compared with a smooth tube, this heat transmission crisis occurs only at a steam mass content of  greater than 0.9, that is to say just before the end of evaporation. This is attributable to the swirl which the flow experiences due to the spiral ribs. Due to the different centrifugal force, the water fraction is separated from the steam fraction and is transported to the tube wall.
The wetting of the tube wall is thereby maintained up to high steam contents, so that there are high flow velocities even at the location of the heat transmission crisis. This results, despite the heat transmission crisis, in relatively good heat transmission and consequently low tube-wall temperatures.
A number of the evaporator tubes of the combustion chamber advantageously have means for reducing the throughflow of the flow medium. In this case, it proves particularly beneficial if the means are designed as throttle devices. Throttle devices may, for example, be fittings in the evaporator tubes, said fittings reducing the tube inside diameter at a point inside the respective evaporator tube. At the same time, it also proves advantageous to have means for reducing the throughflow in a line system which comprises a plurality of parallel lines and through which flow medium can be supplied to the evaporator tubes of the combustion chamber. In this case, the line system may also precede an inlet header system of evaporator tubes capable of being acted upon in parallel by flow medium. In this case, for example, throttle fittings may be provided in a line or a plurality of lines of the line system. Such means for reducing the throughflow of the flow medium through the evaporator tubes make it possible to adapt the throughput of flow medium through individual evaporator tubes to the respective heating of these in the combustion chamber. As a result, in addition, temperature differences of the flow medium at the outlet of the evaporator tubes are kept particularly low in a particularly reliable way.
The side walls of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically arranged steam generator tubes welded to one another in a gastight manner and in each case capable of being acted upon in parallel by flow medium.
Adjacent evaporator or steam generator tubes are advantageously welded to one another in a gastight manner on their longitudinal sides via metal bands, so-called fins. These fins may already be firmly connected to the tubes during the process of manufacturing the tubes and form a unit with these. This unit formed from a tube and fins is also designated as a finned tube. Information regarding the unit may be found in the commonly assigned U.S. Pat. No. 5,662,070, which is herewith incorporated by reference. The fin width influences the introduction of heat into the evaporator or steam generator tubes. The fin width is therefore adapted, preferably as a function of the position of the respective evaporator or steam generator tubes in the continuous-flow steam generator, to a heating profile predeterminable on the fuel-gas side. In this case, the predetermined heating profile may be a typical heating profile determined from experimental values or else a rough estimation, such as, for example, a stepped heating profile. As a result of the suitably selected fin widths, even when various evaporator or steam generator tubes are subjected to greatly differing heating, it is possible to achieve an introduction of heat into all the evaporator or steam generator tubes such that temperature differences at the outlet of the evaporator or steam generator tubes are kept particularly low. Premature material fatigues are reliably prevented in this way. The continuous-flow steam generator consequently has a particularly long useful life.
A number of superheater heating surfaces are advantageously arranged in the horizontal gas flue, which are arranged approximately perpendicularly to the main flow direction of the fuel gas and the tubes of which are connected in parallel for a throughflow of the flow medium. These superheater heating surfaces, arranged in a suspended form of construction and also designated as bulkhead heating surfaces, are heated predominantly by convection and follow the evaporator tubes of the combustion chamber on the flow-medium side. Particularly beneficial utilization of the fuel-gas heat is thereby ensured.
Advantageously, the vertical gas flue has a number of convection heating surfaces which are formed from tubes arranged approximately perpendicularly to the main flow direction of the fuel gas. These tubes of a convection heating surface are connected in parallel for a throughflow of the flow medium. These convection heating surfaces, too, are heated predominantly by convection.
In order, furthermore, to ensure particularly full utilization of the heat of the fuel gas, the vertical gas flue advantageously has an economizer.
Advantageously, the burners are arranged on the end wall of the combustion chamber, that is to say on that side wall of the combustion chamber which is located opposite the outflow orifice to the horizontal gas flue. A continuous-flow steam generator designed in this way can be adapted particularly simply to the burnup length of the fuel. By the burnup length of the fuel is to be meant, here, the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the flame of the fuel. In this case, the maximum burnup length for the respective continuous-flow steam generator is obtained at the steam power output M under the full load of the continuous-flow steam generator, the so-called full-load operating mode. The burnup time tA of the flame of the fuel is, in turn, the time which, for example, a coaldust grain of average size requires to burn up completely at a specific average fuel-gas temperature.
In order to keep material damage and undesirable contamination of the horizontal gas flue, for example due to the introduction of high-temperature molten ash, particularly low, the combustion chamber length defined by the distance from the end wall to the inlet region of the horizontal gas flue is advantageously at least equal to the burnup length of the fuel when the continuous-flow steam generator is in the full-load operating mode. This horizontal length of the combustion chamber will generally amount to at least 80% of the height of the combustion chamber, as measured from the funnel top edge to the combustion chamber ceiling.
For particularly beneficial utilization of the combustion heat of the fossil fuel, the length L (given in m) of the combustion chamber is advantageously selected as a function of the steam power output M (given in kg/s) of the continuous-flow steam generator under full load, of the burnup time tA (given in s) of the flame of the fossil fuel and of the outlet temperature TBRK (given in xc2x0 C.) of the fuel gas from the combustion chamber. In this case, with a given steam power output M of the continuous-flow steam generator under full load, approximately the higher value of the two functions (1) and (2) applies to the length L of the combustion chamber:
xe2x80x83L(M, tA)=(C1+C2xc2x7W)xc2x7tA and
L(M, TBRK)=(C3xc2x7TBRK+C4)W+C5(TBRK)2+C6xc2x7TBRK+C7
with
C1=8 m/s and
C2=0.0057 m/kg and
C3=1.905xc2x710xe2x88x924 (mxc2x7s)/(kgxc2x0 C.) and
C4=0.286(sxc2x7m)/kg and
C5=3xc2x710xe2x88x924 m/(xc2x0 C.)2 and
C6=0.842 m/xc2x0 C. and
C7=603.41 m.
The term xe2x80x9capproximatelyxe2x80x9d as used in this context allows a permissible deviation from the value defined by the respective function of +20%-10%.
The advantages achieved by means of the invention are, in particular, that the suitable choice of the ratio between the steam power output of the continuous-flow steam generator at the full load for a number of evaporator tubes connected in parallel and the inner cross-sectional areas of these evaporator tubes ensures that the throughput of the flow medium through the evaporator tubes is adapted particularly well to the heating and that the temperatures at the outlet of the evaporator tubes are therefore virtually identical. When the continuous-flow steam generator is in operation, the thermal stresses in the containment wall of the combustion chamber, which are caused by temperature differences between adjacent evaporator tubes, remain in this case well below the values at which there is, for example, the risk of tube cracks. It is subsequently possible to employ a horizontal combustion chamber in a continuous-flow steam generator, even with a comparatively long useful life. Moreover, designing the combustion chamber for an approximately horizontal main flow direction of the fuel gas affords a particularly compact form of construction of the continuous-flow steam generator. This makes it possible, when the continuous-flow steam generator is incorporated into a power station with a steam turbine, also to have particularly short connecting tubes from the continuous-flow steam generator to the steam turbine.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fossil-fired continuous-flow steam generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.